add .sr-only-focusable; fixes #12259
update docs to include+use .sr-only-focusable
Merge branch 'master' into sr-only-focusable

Conflicts:
	dist/css/bootstrap.css
Merge branch 'master' into sr-only-focusable
Merge pull request #12679 from twbs/sr-only-focusable

add .sr-only-focusable